A turbofan gas turbine engine is generally located in an aircraft enclosure which is referred to as the nacelle. The nacelle provides a smooth contour around the gas turbine engine. In a confluent flow turbofan gas turbine engine, the nacelle covers entirely or almost entirely the engine.
When operating, the propulsive jet behind a turbofan gas turbine engine forces the nacelle external flow stream lines to bend inwards near the rear thereof. The stream line curvature is associated with a pressure gradient pointing away from the nacelle, such that the aft section of the nacelle is subjected to a low pressure which increases nacelle drag when the aircraft is moving. The jet induced nacelle drag is somewhat directly proportional to the jet core flow rate and the mixing intensity at the jet boundary behind the engine. Nacelle drag can also increase due to noise reduction measures such as chevrons or lobed nozzles, which increase the entrainment rate of the ambient fluid around the nacelle.
Overall, it was desirable to provide a way to mitigate the nacelle drag resulting from the ambient fluid entrainment by the propulsive jet.